In general, a starter switching circuit of an AC motor uses electromagnetic force of a coil, and an electromagnetic relay or an electromagnetic contactor is typically used as the starter switching circuit according to a contact capacity. However, a non-contact solid state relay has been recently used to prevent contact wear using a semiconductor element. A short circuit protecting circuit breaker is connected in series to an overload protector in the starter switching circuit so as to constitute a motor starter. Here, in some cases, an inverter circuit may be replaced with a speed control function in order to improve energy efficiency.
Since a start current of a motor is generally about seven or eight times a rated current, the contact capacity of the switching circuit should be designed to be high according to the start current. Furthermore, it should be considered that starting characteristics are degraded due to an increase in the occurrence of overheating of a motor coil when start/stop is frequently performed. On the contrary, in the case of the solid state relay using a semiconductor contact, the start current can be controlled, but power consumption occurs due to heating of a semiconductor element itself while a motor is driven. Furthermore, when the solid state relay fails and short-circuits, the circuit is highly possibly damaged by a fault current.
Moreover, an overcurrent protector for a motor or a short-circuit current protecting circuit breaker is separated from a switch, causing an additional increase in the cost. However, it is very difficult to simplify a circuit in order to reduce the cost, and, due to the characteristics of a thermal-type protective element, operation characteristics thereof are not stable according to use environments.
In addition, a driving voltage phase control circuit employing a semiconductor element may be simply designed. However, it is difficult to secure energy efficiency achieved through, for example, variable frequency drive, when a variable speed-variable load such as a fan, a pump or a compressor is driven at a rated or lower speed. In particular, it is difficult to control such a load at a low speed.